Essentially, foaming machines receive a mixture of reactants and provide a path in which initial foaming of the reactant mix and subsequent curing to form a self supporting polymer foam can take place. Blocks are then cut and matured.
In one form of machine the polymer foam is taken off vertically. Such machines are successful but require careful operation, not always easy for those familiar with the usual horizontal machines, to ensure that the foaming reaction takes place freely and that the foam develops sufficient early strength for the spiked conveyors used to engage the foam and carry it away.
In the other and by far the commoner form of machine, which is the one the invention is concerned with, the polymer foam is taken away generally horizontally. The machine is essentially a long channel, with reactants fed at one end to travelling webs of paper or plastics and the foam taken off at the other. The machines inherently have a high production rate and are very large, production rates of 100 kg to 500 kg minute and lengths of over 100 meters being common. Capital costs are high, yet very often curing and handling the foam produced is a limiting factor, or markets are modest, and plants are run only part of the day. A smaller, low-production machine would be desirable to make best use of the investment in a plant and produce foam for local markets rather than transporting high-volume relatively low-value products long distances.
The high production rates are however inherent from the nature of the process in current machines. Prior to foaming, the reactants are dense compared to the initially-fluid foam they give rise to. The conveyors taking the foam away, sloping forwards to ensure that foam does not simply escape from the back of the machine, have then to run fast enough to obviate the tendency of younger, high-density material to underrun older, low density material in the forward direction of the conveyor. If materials did underrun, with lighter foam floating on top of denser material, finished foam of uneven properties would be produced, but this is prevented by taking the older material away as fast as the younger material would otherwise run forward under it.
The problem is worsened by the limited steepness of profile that still-fluid foam can sustain. If the limit is exceeded the foam will slump back over younger, denser materials independent of underrunning tendencies as such, and to obviate slumping with a practical height of block, as well as the escape of reactants referred to above, the initial part of the conveyor is invariably sloped downwards in the direction of travel. This increases the tendency for underrunning and hence further increases minimum conveyor speeds.